This invention is concerned with integrated circuits (ICs) and is more particularly concerned with multichannel IC amplifiers.
FIG. 1 is a schematic plan view of a conventional four-channel IC amplifier 10. The multichannel amplifier 10 includes four separate amplifier circuits 12, each corresponding to a respective channel. A pair of input terminals 14 and a pair of output terminals 16 are provided for each channel. Each amplifier circuit 12 is coupled to its respective input terminals 14 via a pair of DC-blocking capacitors 18. A positive-sense capacitor 18a and a negative-sense capacitor 18b are provided for each amplifier circuit 12.
A problem that arises with the conventional multichannel amplifier arrangement as shown in FIG. 1 is crosstalk between adjacent channels resulting from parasitic capacitive coupling between the negative-sense capacitor for one channel with the positive-sense capacitor for an adjacent channel. Even with very low parasitic values, crosstalk can arise because of the high frequency data transmission rates currently employed (e.g., about 1 GHz or greater). Crosstalk can also result from deep substrate coupling.
Isolation of channels is a technique that has been used to reduce crosstalk. For example, deep trench isolation has been used, but is not available in all IC fabrication processes. Isolation by spacing channels from one another can also be used but is not space efficient. Power supply traces can be used to shield the capacitors from each other, thereby reducing edge to edge coupling. However, enough deep substrate coupling may remain in this case to cause significant crosstalk.
It accordingly would be desirable to provide a crosstalk-reduction technique that does not suffer from the disadvantages of known techniques.
According to an aspect of the invention, a multichannel IC amplifier includes a plurality of amplifier circuits formed on an IC substrate. Each amplifier circuit is coupled to respective inputs via a pair of capacitors, and the capacitors are configured so as to substantially equalize like sense and unlike sense coupling between adjacent channels. xe2x80x9cLike sense couplingxe2x80x9d will be understood to mean positive sense capacitor to positive sense capacitor coupling and negative sense capacitor to negative sense capacitor coupling. xe2x80x9cUnlike sense couplingxe2x80x9d will be understood to mean negative sense capacitor to positive sense capacitor coupling and positive sense capacitor to negative sense capacitor coupling.
According to another aspect of the invention, a method of suppressing crosstalk between adjacent channels in a multichannel IC amplifier includes coupling substantially equal crosstalk signals to each capacitor of a pair of capacitors for a given channel. The coupled crosstalk signals may then be rejected as common mode signals at the differential amplifier for the given channel.
According to still another aspect of the invention, there is provided a coplanar arrangement for capacitor plates. Each of the four plates corresponds to a respective DC-blocking capacitor. The four plates respectively correspond to a left-channel positive-sense capacitor, a left-channel negative-sense capacitor, a right-channel positive-sense capacitor and a right-channel negative-sense capacitor. The plate corresponding to the left-channel positive-sense capacitor and the plate corresponding to the right-channel positive-sense capacitor define therebetween a first shared edge region having a unit length U. The plate corresponding to the left-channel negative-sense capacitor and the plate corresponding to the right-channel positive-sense capacitor define therebetween a second shared edge region having a length of two times the unit length, i.e. 2U. The plate corresponding to the left-channel negative-sense capacitor and the plate corresponding to the right-channel negative-sense capacitor define therebetween a third shared edge region having the unit length U.
The present invention employs cancellation of cross-coupled signals to reduce or prevent crosstalk, so that isolation and/or shielding between channels may not be necessary. It is, however, also contemplated to use the cancellation arrangement of the present invention together with isolation and/or shielding.